Article: http://www.cleverworkarounds.com/2013/09/23/how-to-filter-on-a-managed-metadata-column-via-rest-in-sharepoint-2013/
And this article: https://sharepointgotchas.wordpress.com/2014/11/18/using-rest-to-query-data-by-filtering-on-taxonomy-field/
The filtering works using CAML query.